This invention relates to novel alkoxycarbonylphosphonic acids and (alkylthio)carbonylphosphonic acids and their use as plant growth regulants. The use of various carbamoylphosphonates for control of plant growth is known. For example, see U.S. Pat. No. 3,627,507 and German Offenlegungsschrift No. 2,040,367. However, neither of these references suggests the compounds of this invention. Further, some of the active compounds within the scope of this invention are known. U.S. Pat. No. 3,033,891 discloses esters and salts of (alkylthio)carbonylphosphonic acid, but the reference does not suggest the plant growth regulant activity of these compounds.